1. Field of the Invention
The present invention is directed generally to an apparatus for modulating the intensity of a particle beam.
2. Description of the Related Art
A beam blanking system for pulsing an electron beam is disclosed in Rev. Sci. Instrum. 56 (4) April 1985, pages 567 through 571. Beam blanking systems are used in modified scanning electron microscopes and in electron beam testers for, among other things, stroboscopic measurement of high frequency electrical signals in components of microelectronics, such as VLSI circuits. The sampling method is known for use in electron beam testing, see, for example, Microelectronic Engineering, Volume 4, No. 2, 1986, pages 77 through 106 and Volume 5, 1986, pages 523 through 530. To completely measures the time dependency of the signals appearing at interconnects in fast digital circuits by the use of the sampling method, it is often necessary to shift the phase relationship of the electron pulses over a range of 10 through 100 ns as prescribed by the period duration of the signal. Such electron pulses, which are generated with the assistance of a beam blanking system, have a pulse width of 10 through 300 ps. The phase relation is defined by the point in time of keying.
In conventional electron beam testers, the phase shift is usually performed with the assistance of mechanical extension lines, or delay lines, that guarantee a time stable, consistant delay of the trigger signal driving the pulse generator of the beam blanking system. Mechanical phase shifters, however, have the disadvantage that extremely long delay lines are required to delay a signal over a range of time from 10 through 100 ns. Moreover, a considerable attenuation of the trigger signal occurs during the use of long delay lines which results in a noticeable diminution of the phase stability of the arrangement, also known as jitter, that influences the chronological resolution. Furthermore, it is not possible to suddenly discontinuously change the phase relationship of the electron pulses since the length of the extension lines can only be continuously modified.
The use of electronic delay elements for delaying the trigger signal does not come into consideration since these devices do not exhibit the time stability required for measurements of fast digital circuits.